cfn-lambda

Purpose

A simple flow for generating CloudFormation Lambda-Backed Custom Resource handlers in node.js. The scope of this module is to structure the way developers author simple Lambda-Backed resources into simple functional definitions of Create, Update, Delete.

Also supports:

• Extremely simple deployments
• Automatic creation of CloudFormation Quick Launch links, so you can easily share your open-source Custom Resources in any region!
• Automatic expansion of __default__ Properties values into any tree or subtree of any Custom Resource utilizing cfn-lambda for implementation
• Validation of 'ResourceProperties'
  • Using inline JSONSchema objects as Schema
  • Using a SchemaPath to JSONSchema file
  • Using a custom Validate callback
• Optional NoUpdate callback, which runs as a READ function for when Update should be made due to all parameters being identical - because some resources still need to return attributes for Fn::GetAtt calls.
• Convenience Environment values
  • Lambda ARN
  • Lambda Name
  • AWS Account ID for the Lambda
  • Region for the Lambda
• Array of String TriggersReplacement for Resource.Properties key strings that force delegation to resource Create for seamless full replacement without downtime in many cases, and forcing UPDATE_COMPLETE_CLEANUP_IN_PROGRESS.
• An SDKAlias function generator that structures and greatly simplifies the development of custom resources that are supported by the Node.js aws-sdk but not supported by CloudFormation.

This package on NPM
This package on GitHub

Custom Resources To Try

Since version 2.0.0 of this tool, this supports Launch Pages, which are a simple way to share your resources. These are some you can try.

• Add Amazon Lex Slot Types as a supported CloudFormation resource here
• More added soon! Just click a Launch link to install into your account.

Launch Pages

Once you build a resource with this tool, if you use the --public setting, you can share these resources by sharing your Launch Pages. These are HTML pages hosted in S3 that the tool automatically creates during deployments. They're only accessible to the public if you specifically set --public during a deployment. You should only do this for open source custom resource types.

These pages are accessible in all regions in which you deploy resource types to. They generally follow this link pattern: <s3 host>/<your package name>-<your AWS account ID>-<region>/<dash-delimited-version>.html.

If you're confused, check for the HTML pages that are inserted into the S3 buckets deployed by this tool during deploys.

Source Code Examples

The "old" resources below work, they just are now supported by built-in CloudFormation resource types. They still are good examples of how to implement.

• Stable Custom::LexSlotType (GitHub / NPM)
• Stable, old Custom::ApiGatewayRestApi (GitHub / NPM)
• Stable, old Custom::ApiGatewayMethod (GitHub / NPM)
• Stable, old Custom::ApiGatewayMethodResponse (GitHub / NPM)
• Stable, old, uses LongRunning configurations correctly Custom::ElasticSearchServiceDomain (GitHub / NPM)

Call For Contributions

Hey you! Are you an AWS automation engineer? I'd love if you'd author open-source resources with this tool. Just submit a PR to this page for a specific tag on your repository, and I'll review it and add it to the page.

Furthermore, if you want to help style the generated HTML in the launcher pages, I'd love help with that too :)

Feel free to tweet me about involvement too: @ayetempleton thanks!

Deployment of Lambdas

Any custom resource using this tool as a dependency can run deploy scripts from the root of the custom resource project to deploy Lambdas to all regions.

To do this most simply, add this line to the "scripts" section of your package.json inside your repository using this module as a direct dependency:

"deploy": "node ./node_modules/cfn-lambda/deploy.js --allregions --logs"

This will deploy your custom resource to all regions. If you want to customize this behavior, use the options below. These options also apply to using the deploy.js script, as well.

You can also deploy the Lambdas programmatically from JS by importing the module: require('cfn-lambda'). The same options that work on the command line below work as values on an option hash: require('cfn-lambda')(options, callback).

You must also set up:

1. Add <reporoot>/execution-policy.json to define the abilities the Lambda should have.
2. Have AWS credentials configured in your environment, via one of:

• $AWS_PROFILE in your environment
• a credentials file
• $AWS_ACCESS_KEY_ID and $AWS_SECRET_ACCESS_KEY in your environment.

You then run this from within the repository directly depending on cfn-lambda (your custom resource implementation using this package):

  $ npm run deploy

Again, this will, if you used the suggested package.json edit to use the deploy.js inside this repo, deploy your custom resource implementation to all regions using some default settings. Please read the below options, as you may want to restrict deployment to only a couple regions. You might want to do this if your custom resource uses AWS services only available in a smaller subset of regions than Lambda is available in.

Since this uses CloudFormation to install, you can get the ServiceToken usable in the region to begin creating resources from the Outputs and Exports of the generated stack at the ServiceToken key. The stack by default launches with name <your-package-name>-<your package version>. You can manually grab the value from the Outputs in that template, and use it later, or you can use Fn::ImportValue to directly get it in any stack.

    "Fn::ImportValue": "<resource name>-<resource version>-ServiceToken"

This value may change if you use the --alias or --version flags below, since the Stack name will be different.

Options

When using the module in JS, a simple hash is passed in as the first argument. When on the command line, boolean parameters are set to true with --<argname>. Parameters needing a value are set with --<argname> <argvalue>.

account

Used to specify the AWS Account ID to launch the systems into. By default, is the account associated with the Role or User currently invoking the script. Useful when a cross-account role is being used.

alias

Instead of deploying the systems with this naming pattern: <your package name>-<your package version>, it replaces the package name: <specified alias>-<your package version>.

allregions

Causes your custom resource to be deployed on all regions supporting AWS Lambda. false by default.

logs

Makes the deployment system log to STDOUT. Defaults to false when used via JS as a module, and defaults to true on CLI. You can turn CLI logging off with the --quiet option.

module

Sets the tool to deploy the custom service in the module you provide, relative to the current working directory. For example, if this package, and your package using cfn-lambda are both dependencies of a project, from that project's root, set --module <custom resource to deploy>. The --path argument takes precedence if both --module and --path are defined.

path

The tool will deploy the cfn-lambda-based custom resource you have defined to use the provided path. Best used when cfn-lambda is not in your project's node_modules directory. If you do not provide --path or --module, the system assumes that cfn-lambda is in the node_modules directory of your project, and thus uses this --path: cfn-lambda/../../ (assumes you're using cfn-lambda as a normal node_modules dependency of your custom resource project directory).

public

Sets the Quick Launch Page to be publicly accessible, as well as the Lambda function code zip bundle for your custom resource type. Defaults to false.

Setting this to true if you are an open source software author will allow anyone to install your custom resources without needing to run this installation script, just by clicking a link on the browser, in the HTML page this tool generates for you in your S3 buckets in each region you have this script run on. Bear in mind, that you will be responsible for the AWS fees associated with others accessing your bucket.

quiet

Forces logs off, on both CLI and with JS module usage. Takes precedence over --logs, so use this to make your CLI invocations stop producing logs. Defaults to false.

regions

Sets the AWS regions to deploy your custom resource type to. Defaults to the value of $AWS_REGION in your environment, or none, if you do not set $AWS_REGION.

This value is ignored if you set --allregions.

On the CLI, values are passed in comma-delimited, with no spaces, like us-east-1,us-east-2. When using the module via JS, pass this value in as a plain JavaScript array.

rollback

Setting this value to false prevents the CloudFormation stacks this tool uses to deploy from rolling back when any errors occur during initial creation. On the CLI, the value must be exactly false. With module-style usage in JS, any falsey value will achieve the same effect. Defaults to true, thus allowing any stacks with failures during creation to roll back.

version

Instead of deploying the systems with this naming pattern: <your package name>-<your package version>, it replaces the package version: <your package name>-<specified version>. Technically, it does not have to be a number, but using the format x-y-z is strongly suggested.

Usage

This is a contrived example call to fully demonstrate the way to interface with the creation API.

You can manually define these properties, or use SDKAlias for Create, Update and/or Delete.

Resource Lambda Generation

var CfnLambda = require('cfn-lambda');


exports.handler = CfnLambda({

  Create: Create, // Required function
  Update: Update, // Required function
  Delete: Delete, // Required function

  // Any of following to validate resource Properties
  // If you do not include any, the Lambda assumes any Properties are valid.
  // If you define more than one, the system uses all of them in this order.
  Validate: Validate,     // Function
  Schema: Schema,         // JSONSchema v4 Object
  SchemaPath: SchemaPath, // Array path to JSONSchema v4 JSON file
  // end list

  NoUpdate: NoUpdate, // Optional
  TriggersReplacement: TriggersReplacement, // Array<String> of properties forcing Replacement

  LongRunning: <see Long Running below> // Optional. Configure a lambda to last beyond 5 minutes.

});

Environment Convenience Property

Provides convenience Environment values.:

var CfnLambda = require('cfn-lambda');
// After receiving `event` and `context`...
console.log(CfnLambda.Environment);
/*
{
  `LambdaArn`: 'foo bar',      // Full ARN for the current Lambda
  `Region`: 'us-east-1',       // Region in which current Lambda resides
  `AccountId`: '012345678910', // The account associated with the Lambda
  `LambdaName`: 'LambdaName'   // Name for the current Lambda
}
*/

Only works after the generated CfnLambda function has been called by Lambda.

Create Method Handler

Called when CloudFormation issues a 'CREATE' command.
Accepts the CfnRequestParams Properties object, and the reply callback.

function Create(CfnRequestParams, reply) {
  // code...
  if (err) {
    // Will fail the create.
    // err should be informative for Cfn template developer.
    return reply(err);
  }
  // Will pass the create.
  // physicalResourceId defaults to the request's `[StackId, LogicalResourceId, RequestId].join('/')`.
  // FnGetAttrsDataObj is optional.
  reply(null, physicalResourceId, FnGetAttrsDataObj);
}

Update Method Handler

Called when CloudFormation issues an 'UPDATE' command.
Accepts the RequestPhysicalId String, CfnRequestParams Properties object, the OldCfnRequestParams Properties object, and the reply callback.

function Update(RequestPhysicalID, CfnRequestParams, OldCfnRequestParams, reply) {
  // code...
  if (err) {
    // Will fail the update.
    // err should be informative for Cfn template developer.
    return reply(err);
  }
  // Will pass the update.
  // physicalResourceId defaults to pre-update value.
  // FnGetAttrsDataObj is optional.
  reply(null, physicalResourceId, FnGetAttrsDataObj);
}

Delete Method Handler

Called when CloudFormation issues a 'DELETE' command.
Accepts the RequestPhysicalId String, CfnRequestParams Properties object, and the reply callback.

function Delete(RequestPhysicalID, CfnRequestParams, reply) {
  // code...
  if (err) {
    // Will fail the delete (or rollback).
    // USE CAUTION - failing aggressively will lock template,
    //   because DELETE is used during ROLLBACK phases.
    // err should be informative for Cfn template developer.
    return reply(err);
  }
  // Will pass the delete.
  // physicalResourceId defaults to pre-delete value.
  // FnGetAttrsDataObj is optional.
  reply(null, physicalResourceId, FnGetAttrsDataObj);
}

Async function support

If your handler function is async, then your custom lambda resource may unexpectedly exit before your Promise(s) have resolved. The result of this is your CloudFormation stack hanging for up to an hour which is very undesirable behavior. You can read more about the underlying issue in this great medium article. To fix this, you can use the async counterparts of the given handler type:

Create -> AsyncCreate
Update -> AsyncUpdate
Delete -> AsyncDelete
NoUpdate -> AsyncNoUpdate

This difference with the async counterparts is that there is no reply() callback. Instead, your function should return a Promise (or be marked async) which resolves with an object containing the PhysicalResourceId and FnGetAttrsDataObj outputs. See below example:

const wait = () => {
  return new Promise((resolve, reject) => {
    setTimeout(() => {
      resolve();
    }, 2000);
  });
};
const createHandler = async (cfnRequestParams) => {
  await wait();
  return {
    PhysicalResourceId: "yopadope",
    FnGetAttrsDataObj: {
      MyObj: "dopeayope"
    }
  };
};
exports.handler = cfnLambda({
  AsyncCreate: createHandler,
  ...
});

Validating Properties

Used before 'CREATE', 'UPDATE', or 'DELETE' method handlers. The CloudFormation request will automatically fail if any truthy values are returned, and any String values returned are displayed to the template developer, to assist with resource Properties object correction.

Important: To prevent ROLLBACK lockage, the 'DELETE' will be short circuited if this check fails. If this check fails, CloudFormation will be told that everything went fine, but no actual further actions will occur. This is because CloudFormation will immediately issue a 'DELETE' after a failure in a 'CREATE' or an 'UPDATE'. Since these failures themselves will have resulted from a validation method failure if the subsequent 'DELETE' fails, this is safe.

May be a:

• Custom validation function as Validate callback
• JSONSchema v4 Schema
• JSONSchema v4 file path as SchemaPath

Validate Method Handler

The truthy String return value will cause a 'FAILURE', and the String value is used as the CloudFormation 'REASON'.

// Example using a custom function
// CfnRequestParams are all resource `Properties`,
//   except for the required system `ServiceToken`.
function Validate(CfnRequestParams) {
  // code...
  if (unmetParamCondition) {
    return 'You must blah blah include a parameter... etc'
  }
  if (someOtherCondition) {
    return 'Informative message to CFN template developer goes here.'
  }
  // Returning a falsey value will allow the action to proceed.
  // DO NOT return truthy if the request params are valid.
}

Schema Object - JSONSchema Version 4

Using a JSONSchema Schema property value will automatically generate the String invalidation return values for you when validating against the parameters - simply provide the template and the validation and error messging is taken care of for you.

If you choose to use a JSONSchema template, the service will also use the JSONSchema metaschema to ensure the provided JSONSchema is a valid schema itself.

// Example using a custom JSONSchema Version 4 template
// This might be in a file you manually load like `schema.json`, or a JS object.
var Schema = {
  type: 'object',
  required: [
    'foo'
  ],
  properties: {
    foo: {
      type: 'string'
    },
    selectable: {
      type: 'string',
      enum: ['list', 'of', 'valid', 'values']
    }
  },
  additionalProperties: false
};

SchemaPath Array - Path to JSONSchema Version 4 File

A convenient way to get the benefits of Schema object validation, but keeping your code clean and segregated nicely.

The path is defined as an Array so that we can use the path module.

var SchemaPath = [__dirname, 'src', 'mytemplate.json'];

NoUpdate Method Handler

Optional. Triggered by deep JSON object equality of the old and new parameters, if defined.

Even when short-circuiting an Update is a good idea, a resource provider may still need to return a set of properties for use with Fn::GetAtt in CloudFormation templates. This NoUpdate handler triggers in the special case where no settings on the resource change, allowing the developer to simultaneously skip manipulation logic while doing read operations on resources to generate the attribute sets Fn::GetAtt will need.

// Using a custom NoUpdate for READ to supply properties
//   for Fn::GetAtt to access in CloudFormation templates
function NoUpdate(PhysicalResourceId, CfnResourceProperties, reply) {
  // code that should be read-only if you're sane...
  if (errorAccessingInformation) {
    return reply('with an informative message');
  }
  // Can have many keys on the object, though I only show one here
  reply(null, PhysicalResourceId, {Accessible: 'Attrs object in CFN template'});
}

Long Running

This is very advanced Lambda self replication.

The inner workings of this feature are a lot to take in. I strongly suggest you just read the source code for cfn-elasticsearch-domain to see how the index.js file utilizes the LongRunning feature, as the concrete example code is much more understandable than abstract definitions of parameters and options.

cfn-elasticsearch-domain/index.js GitHub

If you have the appetite for it... Read on...

Some resources will take a considerable amount of time to complete, like an Elasticsearch Domain. In order to utilize Lambda-Backed Custom Resources within CloudFormation while avoiding the hard 300 second / 5 minute Lambda timeout for resources that will take more than 5 minutes to finish, cfn-lambda allows resource developers to leverage bundled Lambda self-replication logic. Developers can configure the LongRunning property on the lambda definition options object with a few settings to tell cfn-lambda to simply run some action initialization code (such as initiating an Elasticsearch Domain Create), then periodically self-replicate to check the status of the long-running process. The majority of cases where AWS APIs or SDKs return statusCode === 202 will use this technique to avoid Lambda death at 5 minutes.

The self-replication strategy will trigger if the developer configures the following on the LongRunning property object: PingInSeconds, MaxPings, LambdaApi, Methods.METHOD_NAME.

PingInSeconds

The duration a Lambda will wait between spawning self-replication calls and triggering the next LongRunning.Methods.METHOD_NAME call. This value should not exceed 240 (4 minutes), because we need to leave enough time before the 5 minute hard process death is triggered by AWS.

After this time, the lambda will spawn a new lambda, which will call the LongRunning.Method.METHOD_NAME, where METHOD_NAME is Create, Delete, Update, depending on which are configured and the lifecycle phase the resource is moving through.

MaxPings

The maximum number of self-respawn and check cycles the Lambda will go through. After exceeding this number, the Lambda will circuit break and send a Failed to Stabilize response to the CloudFormation stack.

LambdaApi

cfn-lambda uses this namespace to invoke the Lambda. Allows the Custom Resource developer using cfn-lambda to specify a Lambda API version, or stub the value out for testing.

In most cases, just pass new AWS.Lambda({apiVersion: '2015-03-31'}) as the API namespace.

Methods

Most of the LongRunning logic happens here. At its most configured, this subobject will have 3 properties corresponding to the normal actions: Create, Update, and Delete.

When you configure one of these properties, the flow of that CloudFormation action type changes - within the reply callback function in the corresponding normal/top-level callback you defined for the resource, reply-ing with success just tells cfn-lambda that you correctly initialized the Create/Delete/Update for the resource, and to start using the corresponding LongRunning.Methods.METHOD to ping to final completion. That is, the resource will not COMPLETE the action until the function you define finalizes the SUCCESS.

Read below to see how to define each LongRunning.Methods.METHOD...

LongRunningContext Param Object

All three LongRunning.Methods receive a special object as their first parameter. The LongRunningContext object carries useful state across all spawned lambda ping cycles.

• LongRunningContext.RawResponse: carries the original intercepted call that your first Create initialization call tried to send to CloudFormation. Used internally for state manipulation. DO NOT ALTER THIS VALUE unless you really know what you're doing, as tampering can cause Lambda recursion to spiral out of control!
• LongRunningContext.PhysicalResourceId: Carries the original PhysicalResourceId intercepted call that your first Create initialization call tried to send to CloudFormation. Useful if your check functions need this value and cannot recompute it from the ResourceProperties sent by CloudFormation.
• LongRunningContext.Data: Carries the original data hash, if present, intercepted from your call to reply within the initializer method. Useful if your check functions need these data value(s) and cannot recompute them from the ResourceProperties sent by CloudFormation. Will not be present if you did not pass a third parameter to reply in the initializer, since the GetAtt-usable Data hash is optional in cfn-lambda.
• LongRunningContext.PassedPings: The number of ping spawns before this current run that have occurred. DO NOT ALTER THIS NUMBER! Subtracting from this number will make your Lambdas infinitely self-replicate, very very bad!

LongRunning.Methods.Create

Will be called during Lambda pingspawn cycles. Here, CheckCreate is an example of a check function definition for LongRunning.Methods.Create.

function CheckCreate(LongRunningContext, params, reply, notDone) {
  // LongRunningContext is object type specified above
  // params are Properties straight from CloudFomation
  // reply is callback just like in normal Create,
  //    call it with reply(errMsg) or reply(null, physicalId, AttrHash)
  // notDone takes no parameters, use this to tell
  //   cfn-lambda to use another ping/spawn cycle and check again later
}

LongRunning.Methods.Update

Will be called during Lambda pingspawn cycles. Here, CheckUpdate is an example of a check function definition for LongRunning.Methods.Update.

function CheckUpdate(LongRunningContext, physcialId, params, oldParams, reply, notDone) {
  // LongRunningContext is object type specified above
  // physicalId is PhysicalResourceId from pre-Update resource state
  // params are Properties straight from CloudFomation
  // oldParams are Properties from CloudFormation for before the Update began
  // reply is callback just like in normal Update,
  //    call it with reply(errMsg) or reply(null, physicalId, AttrHash)
  //    to finalize the transition and notify CloudFormation.
  // notDone takes no parameters, use this to denote no errors and tell
  //   cfn-lambda to use another ping/spawn cycle and check again later
}

LongRunning.Methods.Delete

Will be called during Lambda pingspawn cycles. Here, CheckDelete is an example of a check function definition for LongRunning.Methods.Delete.

function CheckDelete(LongRunningContext, physcialId, params, reply, notDone) {
  // LongRunningContext is object type specified above
  // physicalId is PhysicalResourceId from pre-Delete resource state
  // params are Properties straight from CloudFomation
  // reply is callback just like in normal Delete,
  //    call it with reply(errMsg) or reply(null, physicalId, AttrHash)
  //    to finalize the transition and notify CloudFormation.
  // notDone takes no parameters, use this to denote no errors and tell
  //   cfn-lambda to use another ping/spawn cycle and check again later
}

TriggersReplacement Array

Optional. Tells cfn-lambda to divert the 'Update' call from CloudFormation to the Create handler the developer assigns to the Lambda. This technique results in the most seamless resource replacement possible, by causing the new resource to be created before the old one is deleted. This Delete cleanup process occurs in the UPDATE_COMPLETE_CLEANUP_IN_PROGRESS phase after all new resources are created. This property facilitates triggering that said phase.

exports.handler = CfnLambda({
  // other properties
  TriggersReplacement: ['Foo', 'Bar'],
  // other properties
});

Now, if the Lambda above ever detects a change in the value of Foo or Bar resource Properties on Update, the Lambda will delegate to a two-phase Create-new-then-Delete-old resource replacement cycle. It will use the Create handler provided to the same CfnLambda, then subsequently the prodvided Delete if and only if the Create handler sends a PhysicalResourceId different from the original to the reply callback in the handler.

SDKAlias Function Generator

Structures and accelerates development of resources supported by the aws-sdk (or your custom SDK) by offering declarative tools to ingest events and proxy them to AWS services.

Will automatically correctly ignore ServiceToken from CloudFormation Properties. All settings are optional, except api and method.

Usage Reference

var AWS = require('aws-sdk');
var AnAWSApi = new AWS.SomeNamespace();
var CfnLambda = require('cfn-lambda');
// Then used as the Create property as defined in Usage above
var MyAliasActionName = CfnLambda.SDKAlias({ // Like Create, Update, Delete
  returnPhysicalId: 'KeyFromSDKReturn' || function(data) { return 'customValue'; },
  downcase: boolean, // Downcase first letter of all top-level params from CloudFormation
  api: AnAWSApi, // REQUIRED
  method: 'methodNameInSDK', // REQUIRED
  mapKeys: {
    KeyNameInCfn: 'KeyNameForSDK'
  },
  forceBools: [ // CloudFormation doesn't allow Lambdas to recieve true booleans.
    'PathToCfnPropertyParam',     // This will coerce the parameter at this path.
    'Also.Supports.Wildcards.*',
    'But',
    'only.at.path.end'
  ],
  keys: [ // Defaults to including ALL keys from CloudFormation, minus ServiceToken
    'KeysFrom',
    'CloudFormationProperties',
    'ToPassTo',
    'TheSDKMethod',
    '**UsedBeforeMapKeys**'
  ],
  returnAttrs: [
    'KeysFrom',
    'SDKReturnValue',
    'ToUseWithCfn',
    'Fn::GetAttr',
    'You.Can.Access.Nested.Properties.As.Well'
  ],
  ignoreErrorCodes: [IntegerCodeToIgnore, ExWouldBe404ForDeleteOps],
  physicalIdAs: 'UsePhysicalIdAsThisKeyInSDKCall',
  // physicalIdAs: 'OrUseNested.Property.Using.Dot.Notation'
});

// Then...

exports.handler = CfnLambda({
  Create: MyAliasActionName, // Doesn't have to be Create, can be Update or Delete
  // ...
});

Defaults

Sometimes it is advantageous to be able to reuse JSON objects or fragments of JSON objects in Properties of Custom Resources, like when you need to build similar complex/large resources frequently that differ by only a few properties.

Any module using cfn-lambda supports __default__ property expansion. __default__ can be added anywhere in the Properties object for a resource, with __default__ containing an arbitrary JSON/String/Array/null/Number value serialized using toBase64(JSON.stringify(anyObject)). cfn-lambda will expand these properties before hitting any validation checks, by running JSON.parse(fromBase64(encodedDefault)) recursively, and overwriting any values in the __default__ tree with those actually set on the Properties object.

The best example of this is the cfn-variable module's example.template.json, wherein a very large RestApi is created with over a large repeated subtree of Resource objects. cfn-variable is a custom resource that takes any value and serializes it using toBase64(JSON.stringify(anyValue)), making it a perfect fit for this behavior.

In the example in cfn-variable, this technique is used to create 120 Resource objects in under 15 seconds (this example uses less):

// This is cfn-variable storing the serialized object:
"MySubtreeVariable": {
  "Type": "Custom::Variable",
  "Properties": {
    "ServiceToken": {
      "Fn::Join": [
        ":",
        [
          "arn",
          "aws",
          "lambda",
          {
            "Ref": "AWS::Region"
          },
          {
            "Ref": "AWS::AccountId"
          },
          "function",
          {
            "Ref": "VariableCustomResourceName"
          }
        ]
      ]
    },
    "VariableValue": {
      "ChildResources": [
        {
          "PathPart": "a",
          "ChildResources": [
            {
              "PathPart": "aa",
              "ChildResources": [
                {
                  "PathPart": "aaa"
                },
                {
                  "PathPart": "aab"
                },
                {
                  "PathPart": "aac"
                }
              ]
            },
            {
              "PathPart": "ab",
              "ChildResources": [
                {
                  "PathPart": "aba"
                },
                {
                  "PathPart": "abb"
                },
                {
                  "PathPart": "abc"
                }
              ]
            },
            {
              "PathPart": "ac",
              "ChildResources": [
                {
                  "PathPart": "aca"
                },
                {
                  "PathPart": "acb"
                },
                {
                  "PathPart": "acc"
                }
              ]
            }
          ]
        }
      ]
    }
  }
},
// Then this will make the tree 3x because you used a variable with __default__
"ExpandedResourceTree": {
  "DependsOn": [
    "MyRestApi",
    "MyVariable"
  ],
  "Type": "Custom::ApiGatewayResourceTree",
  "Properties": {
    "ServiceToken": "<the token>",
    "RestApiId": {
      "Ref": "MyRestApi"
    },
    "ParentId": {
      "Fn::GetAtt": [
        "MyRestApi",
        "RootResourceId"
      ]
    },
    "ChildResources": [
      {
        "PathPart": "alpha",
        "__default__": {
          "Fn::GetAtt": [
            "MySubtreeVariable",
            "Value"
          ]
        }
      },
      {
        "PathPart": "beta",
        "__default__": {
          "Fn::GetAtt": [
            "MySubtreeVariable",
            "Value"
          ]
        }
      },
      {
        "PathPart": "gamma",
        "__default__": {
          "Fn::GetAtt": [
            "MySubtreeVariable",
            "Value"
          ]
        }
      }
    ]
  }
}